Linear polyesters especially polethylene terephthalate have been traditionally used for the production of textile filaments and fibers. In addition, there is a great market for and different uses of polyesters in the film industry. For example, linear polyesters are used as films for packing and packaging magnetic tapes, photographic films, strappings, electrical insulation, drawing paper, etc. When films are used for packing, in most cases, the film has no direct contact with foods. The shrinking properties and the mechanical strength of polyester films which have been used to wrap foods have been used in combination with other materials, such as paper or polyethylene films.
In the particular case of containers, such as bottles, which are designed to contain liquids such as pop or beer, which contain carbonic acid, linear polyesters are preferred material of construction.
It is known that every use demands special properties of the applied raw material. Further, it is known that special applications can impose special requirements.
One prerequisite for polyesters to meet when used for food packing is an optimal homogeneity for the polymer. Other characteristics, such as the viscosity of the product and the average molecular weight must be chosen carefully since these properties have an influence and relation to important properties such as the mechanical strength or the crystallinity. For many purposes, such as for the production of bottles, an optimal transparency and minimal yellow color in the polymer is required. Generally, properties which for any given purpose are of utmost importance can be absolutely secondary for another use.
For use as a packing material for food or beverage, a further factor is of utmost importance. This factor is that the polymer must not impart any taste or odor to the food product. It is known that due to the degradation or decomposition, polyesters form acetaldehyde which is noticed even in smallest concentration by its strong characteristic odor.
If acetaldehyde is present, it is an indication that there is some degradation which ultimately leads to color formation. Color formation is detrimental, i.e. the polymer must have a minimum of color. The requirement for bottles which shall contain beverages is that the concentration of acetaldehyde which diffuses from the walls of the bottles into the liquid must be exceptionally low. The tolerable limits depend on different factors such as the kind of beverage, shelf-life, etc. but also the methods used to determine the presence of acetaldehyde.
Acetaldehyde can be eliminated partially but not quantitatively from a polyester melt. For example, acetaldehyde can partially be eliminated if a stirred melt is treated at higher temperatures under vacuum. By this method, the tolerable minimum limits are not reached because the temperature of the melt is much too high and, therefore, parallel to the elimination of acetaldehyde previously produced, degradation occurs reforming additional acetaldehyde. Further, the elimination of acetaldehyde in a technically sound installation in economically feasible times more difficult as viscosity increases and the melt layer thickens.
Machines which produce bottles, other hollow bodies or other packings from polyesters can be fed with melt from a condensation reactor. In most cases, these machines will be fed with polyester chips. The plant site selection and transportation problems, however, determine the choice of feed to bottle making machines. Even if a melt is substantially free of acetaldehyde and, chips thus produced are subsequently melted again in order to produce bottles, acetaldehyde is again formed by degradation. The concentration of acetaldehyde in the melt used for the production of bottles depends on the conditions of the remelting of the chips. In any case, the concentration or content of acetaldehyde is unbearably high. Therefore, the amount of acetaldehyde which can diffuse into a liquid which is in contact with the bottles is also too high.
In addition, until recently, there has been no standard testing method for acetaldehyde in bottle polymers or in bottles themselves. The methods applied rely on chromatography (J. A. Myers, J. Chromatog. 99, 709-720 (1974)). The differences between different testing methods now applied by different institutes or individuals are found in the procedures used and in the manner in which the amount of acetaldehyde in the polymer is calculated.
Once the problem of acetaldehyde in polymers was known, many attempts to overcome this problem have been made. The goal was to find conditions under which a renewed formation of acetaldehyde during the remelting of chips in order to form bottles can be minimized. An assumption made in most cases was that the raw material for the production of bottles was polyester chips and that the production of these chips was done at a different site than the production of bottles.